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compared and the bonding strength of the EN plating to the substrate was also measured. The results showed that the EN plating could easily take

place on the intermediate catalytic layer, directly on which a smooth and compact NiP alloy layer without obvious flaws, about 20 mm thickness,

ity, and good solderability [8], as well as that other metallic

become a self-sustaining process only if the so called Pd-

which could provide a good autocatalytic function for EN

plating and a seal function for anodized film. This

novel process achieved acceptable bonding strength and

Available online at www.sciencedirect.com

Applied Surface Science 254 (industries [1]. This makes protective surface treatment an

essential and practicable part of the manufacturing process

for Mg alloys. Among various surface treatments, anodic

oxidization, or plasma electrolytic oxidation (PEO), was

widely adopted to enhance the corrosion resistance of Mg

alloys [25]. But a single anodized film generally was not

adequate for protection of magnesium alloys. So electroless

nickel (EN) plating was adopted as a protective top layer on

the anodized film [6,7], due to its special advantages of

EN coatings, such as uniform deposition, good corrosion

and wear resistance, good electrical and thermal conductiv-

strictly prolix operation process and a high material cost

[10].

It was reported that a Pd-activation process was also adopted

for electroless plating of NiB alloy on TiB2 powders to

improve its spraying performance [11]. However EN plating of

TiB2 powders was found to be an autocatalytic process in our

study. The fact clearly implies that TiB2 may act as a catalyst

for the EN plating process.

Therefore, this paper dealt with a novel palladium-free

activation process for EN plating on anodized magnesium

alloy with a thin intermediate layer containing TiB2 powders,such as high specific strength and stiffness, but the poor

corrosion resistance restricted their large-scale application inactivation technique is adopted [6,7]. However, the Pd-

activation technique was inconvenient in practice due to theMagnesium alloys have a number of desirable properties,2

of about 11 MPa between the substrate and the catalytic layer. An obvious passivation range and higher Ecorr (0.323 V) for the EN plating duringanodic polarization in 3.5 wt.% NaCl solution, implied a typical character of a compact NiP alloy layer, with an effective protection for the

substrate.

# 2008 Elsevier B.V. All rights reserved.

Keywords: Magnesium alloy; Electroless nickel plating; Palladium-free activation; Catalytic; Corrosion resistance

1. Introduction plating might further be deposited onto the EN plating

[9]. Traditionally an EN plating on an anodized film maywas successfully deposited. The catalytic function was principally from TiB powder. The adhesive tensile test indicated a good bonding strengthA novel process for electrole

magnesi

Shuo Sun a,b, Jianguo Liu a,*,a State Key Laboratory for Corrosion and Protection, In

62 Wencui Road, Shb School of Science, Shenyang Universit

Received 22 November 2007; received in revise

Available online

Abstract

In this paper, a novel palladium-free activation electroless nickel (E

was used as catalyst, was introduced for anodized magnesium alloy A* Corresponding author. Tel.: +86 24 23921875; fax: +86 24 23893624.

E-mail address: jgliu@imr.ac.cn (J. Liu).

0169-4332/$ see front matter # 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.apsusc.2008.01.169nickel plating on anodized

alloy

huanwei Yan a, Fuhui Wang a

ute of Metal Research, Chinese Academy of Sciences,

ang 110016, China

f Technology, Shenyang 110023, China

rm 30 January 2008; accepted 30 January 2008

February 2008

plating process, by which a TiB2 powders contained intermediate film

1D. The corrosion behavior of AZ91D without and with coating was

www.elsevier.com/locate/apsusc

2008) 50165022corrosion resistance of a composite coating for the Mg alloy

substrate.

2. Experiment

2.1. Specimen preparation

Specimens were cut from die cast magnesium alloy AZ91D

with a size of 25 mm 25 mm 10 mm. The specific compo-sition of the test materials was listed in Table 1. A hole of 2 mm

in diameter was drilled in the middle of one edge of each

Table 1

Nominal composition of AZ91D (in wt.%)

Al 8.59.5

Zn 0.500.90

Cu

environments. The major elements of the anodizing film,

Fig. 1. Schematic illustration of adhesiv

S. Sun et al. / Applied Surface Sc5018integrating the component of anodized bath, were P, F, Na, Mg,

Cr and O (Fig. 2(b)). A rough surface of the anodized filmwould

be beneficial for the adhesion with the catalytic film followed.Fig. 2. SEM image (a) and EDX spectrum (b) for anodized film on Mg alloy.The average size of TiB2 powders used in the experiment

was about 5 mm (Fig. 3). The adoption of TiB2 as the mainactive source was based upon its autocatalysis for EN

plating. In fact, when the powders were put into the EN

plating bath, a quick reaction of NiP deposition was

observed.

e tensile test for bonding strength.

ience 254 (2008) 50165022The intermediate catalytic film consisted of TiB2 powders

and organic adhesive was designed to apply onto the anodized

film. The catalytic layer presented a rough surface (Fig. 4a and

b), which was helpful for a good adhesion with NiP layer

followed. The element of C, O and Si were primarily from the

organic adhesive (Fig. 4(c)). Some TiB2 powders, or parts of the

single TiB2 particle, were exposed on the surface of the

catalytic film to act as actively catalytic sites for nucleation of

EN plating.

At the initial stage of EN plating for 04 min, a nucleation

process was actually occurred (Fig. 5). Some small granule

particles with Ni and P (Fig. 5(c)) deposited on the catalytic

film surface after 4 min EN plating (Fig. 5(a)). The nucleation

process was preferentially initiated on the exposed powders

(Fig. 5(b)). Those regions, with powders enwrapped by epoxy

adhesive, had no grains deposition (bigger squared area B,

Fig. 5(d)).

After 20 min of EN plating, the growth of NiP deposit film

was clearly observed (Fig. 6). The NiP film extended both

vertically and laterally on the whole surface of the catalytic

intermediate film. Consequently, a compact and defect-free EN

plating layer without distinct cavities and crevices, was

obtained after 120 min (Fig. 7(a)). The appearance of cauli-

flower-like nodule was typical of amorphous materials [13].

The outer layer was verified as the NiP coating containing 9.6

mass% phosphor (Fig. 7(b)).

The cross-sectional morphology of the NiP film/inter-

mediate catalytic film/AZ91D system after 120 min EN plating

was shown in Fig. 8, where no obvious defects could be

observed in the substrate, catalytic film or EN plating as well as

their interfaces. The thickness of the EN plating was about

20 mm. The deposition rate 10 mm/h of the EN plating wasclose to that of the traditional EN plating [14], but somewhat

slower than that of the Pd-activation process on PEO film

(about 25 mm/h [6] and 18 mm/h [7]). The reason might be thatthere was no enough TiB2 powders exposed to the surface of the

intermediate film in the present case. A teeth-like interface

between catalytic film and NiP layer suggested a good

adhesion between them. The adhesive tensile test showed a

bonding strength about 11 MPa for the coating to the substrate

and the detachment was occurred at the interface of anodized

film with the AZ91D substrate. So the bonding strength

between Ni and P layer and intermediate catalytic film was

higher than 11 MPa.

It was reported that the autocatalytic reaction for nickel

deposition on metal substrate (such as Ni) or Pd-active system

was initiated by catalytic dehydrogenation of the reducing

agent. The reactions taking place were as follows [1316]:

H2PO2 H2O !

catalyst heatH HPO23 2Habs (1)

Ni2 2Habs ! Ni 2H (2)H2PO2

Habs ! H2O OH P (3)Then the NiP layer acted as catalytic sites to absorb

hydrogen atoms for the subsequent EN plating. Thus the

Fig. 3. SEM image of TiB2 powders.

S. Sun et al. / Applied Surface Science 254 (2008) 50165022 5019Fig. 4. SEM images of showing surface morphology of a specimen with catalytic film before EN plating (a) and (b) and corresponding EDX spectrum (c).

S. Sun et al. / Applied Surface Sc5020catalysis function of the TiB2 powders might be similar to that

of Ni- or Pd-active system. TiB2 powders could provide high

density nucleation sites for the subsequent EN plating with

enough exposed powders surface. But the real catalytic

mechanism of TiB2, such as the influence of potential range or

the inducement of element, is still under study and will be

published in the following paper.

Fig. 6. SEM images of the specimen after 20 min EN plating.

Fig. 5. SEM images of showing morphology of specimen after 4 min EN plaience 254 (2008) 50165022Fig. 9 showed the potentiodynamic polarization curves of

the bare AZ91D magnesium alloy and the AZ91D alloy with

anodized film, catalytic layer and EN plating, respectively. The

parameters of the potentiodynamic polarization were summar-

ized in Table 5. From Fig. 9, it was observed that the corrosion

current density (Icorr) decreased by one order of magnitude for

the specimen after anodic oxidization. The corrosion potential

(Ecorr) of the anodized film showed little shift to the positive

direction in comparison with that of the bare AZ91D. So the

corrosion resistance of the AZ91D alloy was improved to some

extent by anodic oxidization.

When the anodic oxide film was coated with catalytic layer,

the corrosion potential shifted further to more positive

(0.505 V) and the corrosion current density (Icorr) decreasedsignificantly. This probably resulted from the formation of a

dense insulated barrier layer by intermediate catalytic film

treatment. After EN plating, NiP layer showed a much higher

corrosion potential (0.323 V), which was close to that of thetraditional and Pd-activation process [6,7]. Meanwhile, the

corrosion current density dropped down to about 107 A cm2,indicating less porosity and better corrosion resistance of the

composite coating system. This value was also close to that of

Pd-activation process. The passivation rangewas about 400 mV

ting (a) and (b) and corresponding EDX spectrum (c) area A (d) area B.

S. Sun et al. / Applied Surface Sc(from 0 to 0.4 V), which was different from the results of

literatures [6,7]. There was no obvious passivation in Ref. [6]

but an about 800 mV passivation range existed in Ref. [7]. This

difference may be due to the different characteristic of substrate

PEO film. So, from these parameters, an effective protection of

Fig. 7. SEM image (a) and corresponding EDX spectrum (b) of specimen after

120 min EN plating.

Fig. 8. Cross-sectional micrograph of the specimen after EN plating for

120 min (back scattering electron image).Fig. 9. Potentiodynamic polarization curves of different specimens in 3.5%

NaCl solution.

Table 5

Parameters of potentiodynamic polarization tests of different specimens in 3.5%

NaCl solution

Sample Ecorr (V) Icorr (A cm2) Rp (V cm

2)

Bare (AZ91D) 1.564 1.765 104 1.44 102Anodized film 1.530 1.658 105 1.57 103Catalytic layer 0.505 2.13 106 1.22 104EN plating 0.323 2.72 107 9.61 104

ience 254 (2008) 50165022 5021the composite coating system could be expected for Mg alloy

substrate.

4. Conclusions

A novel palladium-free activation EN plating process was

established on the anodized magnesium alloy. The EN plating

obtained was compact and no obvious flaws. TiB2 powders

showed an autocatalysis function for EN plating. The

deposition rate, 10 mm/h, of the new process was close tothat of the traditional process but somewhat lower than that of

Pd-activation process on PEO film. The corrosion current

density was about 107 A cm2 and the corrosion potential was0.323 V for the new coating systems. Together with theobserved passivation range in the potentiodynamic polarization

curves, the electrochemical parameters revealed an effective

protectiveness of the coating system for Mg alloy. The

relatively good bonding strength about 11 MPa between the

substrate and the catalytic layer suggested a further application

in practical industry. This process also provided a new method

for EN plating on the surface of high reactive metal as well as of

inert materials such as plastic.

Acknowledgments

This work supported by the National Basic Research

Program of China and the National High-Tech Research and

Development Program of China (No. 2006AA03Z538). Thanks

to Prof. Wu for the revision of the paper.

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S. Sun et al. / Applied Surface Science 254 (2008) 501650225022

A novel process for electroless nickel plating on anodized magnesium alloyIntroductionExperimentSpecimen preparationAnodizingCatalytic layer applicationElectroless platingAnalysis methods

Results and discussionConclusionsAcknowledgmentsReferences